Most of Head of the Class took place in the classroom.
Watch out for the window on the stairway sometime - impossible, given that the exteriors show their house in the middle of a row.
When you think about it, even the new big expensive shows don’t always have a lot of sets. Seems like for a while there every season of House added a hospital set - remember when the balcony suddenly appeared and was in every episode? But besides people’s apartments, which kind of look all alike, and places they go to outside the hospital, there’s just House’s office with the big table, one patient room that looks different and sometimes it’s a clean room, Wilson’s office, Cuddy’s office, the clinic which seems to be actually next to the atrium with the elevators and all, and some hallway. And a fake-ass lab and MRI. Considering how fancy the show is, it’s not really much.
Lost in Space didn’t have many: Jupiter 2 exterior, Jupiter 2 interior main deck, planet surface (repeatedly redressed and reused). The Jupiter 2 lower deck and stuff like the Chariot and Pod interiors were used so infrequently, I have a hard time believing they kept them as standing sets.
For a short time back in the 90s, Comedy Central had a sketch show called “Limboland” that had no sets. All the sketches were done on a white background.
Seems to me that 90% of Married With Children was the Bundy living room and maybe the shoe store and occasionally Marcy’s house.
I’ve only ever seen one episode of Sanford and Son, but it (a) was just a few days ago and (b) featured Lamont getting an item appraised by an antique dealer (at the dealer’s shop) before putting it up for auction (at an upscale auction house).
They bought the house next door and had it torn down. 
Three’s Company: The apartment, The Roper’s/Mr. Furley’s apartment and The Regal Beagle.
I have a book of “blueprints” for famous sitcom homes. The creator tried to come of with floorplans based on what we saw on TV. They don’t reconcile well with reality. For example based on the stairway it seems that the entire second floor of the Cunninghams’ house (Happy Days is actually located over their backyard instead of above the first floor. :smack:
The exterior of the building - specifically the stairs. Jack’s Bistro was a common set at the end. Before that, Angelino’s.
I forgot the break room set, so it couldn’t have all been contiguous, as the break room would be downstage of the elevator.
I recall seeing Larry’s apartment a few times.
Didn’t Mystery Science Theater 3000 only have 2 sets? The theater lobby where Joel and the bots talked, and the theater with the screen?
“The Office” seems to mainly take place in, well, the office. I assume from lack of studio audience that they just built an office somewhere and it’s all one set.
Here’s a virtual tour that’s kinda neat. Has video clips if you click on stuff like the fridge or microwave.
Not to mention that “the apartment” set was at least 3 sets: the living room, the bathroom, one or the other of the bedrooms (maybe both, I don’t remember well).
And I think I remember that sometimes there was a dining room and a kitchen.
They also had a set or three for whichever mad scientist was controlling Joel or Mike that season.
In later seasons there were many more sets on MST3K. The ape world, the Roman world, Castle Forester and lots more.